Generally, from the view point of intended pharmacological activities, side-effects and the like, pharmaceutical compounds having an asymmetric center in the molecule are desirable to be used in their optically active form instead of in racemic form. A racemic naphthalene compound (Patent literature 1) represented by general formula [A-2]:
which is known to possess cAMP-specific phosphodiesterase (PDE4) inhibitory activity and be useful as anti-asthma drugs and the like, has one asymmetric carbon atom in the molecule, and therefore it is considered that the compound is desirable to be applied to clinical use in the optically active form. It is known that the compound [A-2] can be obtained by reacting a compound represented by formula [B-1]:
with 4(1H)-quinolinone compound represented by formula [B-2]:
and then reducing the reaction product with sodium borohydride (Patent literature 1). However, the corresponding optically active form per se or a method for preparing the same (optical resolution methods of racemic form, asymmetric synthesis methods and the like) has not been reported so far.
From the viewpoint of synthetic chemistry, upon preparing an optically active form of the compound [A-2], there is considered a method of using an optically active cyclic alcohol compound (an optically active tetrahydroquinoline compound) represented by general formula [Ib]:
[wherein Z represents a protecting group for hydroxyl group, and * represents an asymmetric carbon atom.]    as a synthetic intermediate, instead of using the compound [B-2]. However, the optically active cyclic alcohol compound per se is a novel compound, and of course a method for preparing the same has not been reported so far. Under the circumstances, in order to establish a method for preparing an optically active form of the compound [A-2] comprising using the above-mentioned optically active cyclic alcohol compound, it is required to develop a method for preparing the optically active cyclic alcohol compound with high optical purity and good yield.
Generally, as a method for preparing optically active alcohol compounds, for example, the following methods may be assumed: (1) a method comprising subjecting the corresponding prochiral ketone compound to asymmetric reduction in the presence of optically active oxazaborolidine compounds (CBS catalyst) (Nonpatent literatures 1 and 2); (2) a method comprising subjecting the corresponding prochiral ketone compound to asymmetric reduction in the presence of an asymmetric transition metal complex obtained from a transition metal compound and an asymmetric ligand (Nonpatent literature 3); or (3) a method comprising subjecting the corresponding prochiral ketone compound to asymmetric acylation with lipase (Nonpatent literature 4), etc. However, with regard to cyclic ketone compounds containing a carbonyl carbon and a hetero atom (nitrogen atom) in the same ring moiety such as tetrahydroquinolinone, consideration of possible application of asymmetric reduction or asymmetric acylation methods as mentioned above has not been reported so far.
The present inventors have been earnestly studied and as a result, they have found that an optically active alcohol compound [I] can be prepared with high optical purity and good yield from the corresponding cyclic ketone compound [II] by using an asymmetric reduction catalyst such as CBS catalyst, and completed the present invention.
[Patent literature 1] European Patent No. 748805 (see page 2)
[Nonpatent literature 1] E. J. Corey et al., Journal of The American Chemical Society, Vol. 109, pp. 7925-7926 (1987) (see page 7925)
[Nonpatent literature 2] G. J. Quallich et al., Tetrahedron Letters, Vol. 34, No. 5, pp. 785-788 (1993) (see page 785)
[Nonpatent literature 3] S. Hshiguchi et al., Journal of the American Chemical Society, Vol. 117, pp. 7562-7563 (1995) (see page 7562)
[Nonpatent literature 4] J. Uenishi et al., Journal of Organic Chemistry Vol. 63, pp. 2481-2487 (1998) (see page 2482, lines 18 to 25 at left column)